Some Chrome users reported persistent problems Monday related to Microsoft’s Security Essentials and Forefront security products blocking–and in some cases, deleting–copies of the Google Chrome browser after labeling it as a “severe” threat. Microsoft had released an emergency update for the problem on Friday.

According to an update announcement from Microsoft, its products began “incorrect detection of Google Chrome as PWS:Win32/Zbot,” which is another name for the password-stealing Trojan application known as Zeus, which is designed to harvest people’s financial data.

Webcasts

Videos

Microsoft said the anti-Chrome behavior lasted a few hours before it pushed an update–Security Essentials signature version 1.113.672.0–to fix the issue. “Affected customers should manually update Microsoft Security Essentials with the latest signatures,” by selecting “update” from the software’s update tab, said Microsoft. “After updating the definitions, reinstall Google Chrome. We apologize for the inconvenience this may have caused our customers.”

Google engineering manager Mark Larson said Friday that the Chrome team was aware of the issue and working on an automated fix. “We are releasing an update that will automatically repair Chrome for affected users over the course of the next 24 hours,” he said. But he also cautioned that not all Microsoft Security Essentials users were affected, saying that “if Chrome is working correctly for you, then there’s no need to take any action.”

That was after about 300 people had flooded a Google Chrome help forum, with many saying that after Microsoft Security Essentials (MSE) reported that a virus had been removed, they found their browser missing too. “MSE all of a sudden detected Chrome as a threat and deleted it. Bye bye saved passwords, extensions, and history? Microsoft conspiracy theories are going to run at an all-time high,” read one post.

Another poster said that after reinstalling Chrome multiple times, only to see MSE keep nuking it, he’d temporarily given up. “I finally just removed it and am using Firefox right now. Hopefully they can fix this soon.” On Monday, however, some users were still reporting reinstallation issues.

The irony, of course, is that the only “severe” threat posed by Google Chrome is to Microsoft’s browser market share. Chrome continues to eat into Internet Explorer’s user base, and is now the world’s third-most-used browser, commanding 16% browser market share. That puts it behind Internet Explorer (54%) and Firefox (22%), and ahead of Safari (5%), and Opera (2%), according to Net Applications.

Zeus, meanwhile, is one of the world’s best-known crimeware packs–helped in part by savvy blackmarket-focused marketing campaigns–and has been the tool of choice for a number of international gangs. After becoming one of the most high-end crimeware toolkits on the market, however, Zeus’s developer apparently donated the source code to former rival SpyEye, which last year released a Zeus-SpyEye hybrid.

Earlier this year, however, the source code was leaked online, and has since been appearing in a greater range of malware, after apparently still being actively developed. In one recent incarnation, for example, a version of Zeus known as Zitmo (for “Zeus in the mobile”)–versions of which had already targeted BlackBerry, Symbian, and Windows Mobile devices–began attempting to infect Android devices to steal financial data.

Security professionals often view compliance as a burden, but it doesn’t have to be that way. In this report, we show the security team how to partner with the compliance pros. Download the report here. (Free registration required.)